Lenticular sheets are used to give images an appearance of depth. More specifically, a lenticular sheet comprises a transparent upper layer A having narrow, parallel lenticulas (semi-cylindrical lenses) B on an outer surface, and an image-containing substrate layer C which projects images through the lenticulas. (See FIG. 1A). The two layers of a lenticular sheet provide an image such that different portions of the image are selectively visible as a function of the angle from which the lenticular sheet is viewed. If the image is a composite picture made by bringing together into a single composition a number of different parts of a scene photographed from different angles, and the lenticulas are vertically oriented, each eye of a viewer will see different elements and the viewer will interpret the net result as a three dimensional (3-D) image. The viewer may also move his head with respect to the lenticular sheet thereby observing other views with each eye and enhancing the sense of depth.
Another method for showing 3-D images is the use of a blocking line screen positioned at a specific distance from the composite picture. This process, known as a parallax process, causes blocking of all images except one specific image. This allows the eyes to view different images as three-dimensional (3-D) images, when the blocking line screen is oriented vertically.
When the lenticulas or the blocking line screen is oriented horizontally, each eye receives the same image. In this case, the multiple images give illusion of motion when the composite image is rotated about a line parallel to the viewer's eyes. Thus, a simulation of motion is achieved by the process of tipping the lenticular sheet or the blocking line screen, or by movement of the viewer's head to a different angle with respect to the lenticular sheet.
Whether the lenticulas or the blocking line screen is oriented vertically or horizontally, each of the viewed images is generated by lines of images (also referred to as image lines) which have been interlaced at the spatial frequency of the lenticulas or the blocking line screen. Interlacing lines of each image with other images is referred to as interdigitation. A full set of such interdigitated image lines forms a lenticular image. Interdigitation can be better understood by using an example of four images used to form a composite image with a lenticular sheet that has three lenticulas. In this example, line 1 from each of the four images is in registration with the first lenticula; line 2 from each of the four images is in registration with the second lenticula; etc. Each lenticula is associated with a plurality of image lines D or an image line set (See FIG. 1A), and the viewer sees only one image line of each set with each eye for each lenticula. It is imperative that the image line sets be registered accurately with respect to the lenticulas, so that the proper picture is formed when the assembly is viewed.
One method of conventional recording of the interdigitated image lines requires recording of the interdigitated image lines on a recording material contained on the substrate layer C and then attaching the substrate layer C to the upper layer A, with the recorded image lines D in precise alignment to the lenticulas B to yield the desired image structure. The precise alignment of the specific lenticulas with the desired image line set during the attachment of the recording material to the lenticular overlay is difficult to achieve. This results in a degraded image quality.
Conventional recording of lenticular images has been accomplished with a stereoscopic image recording apparatus that uses optical exposure. A light source, such as a halogen lamp, is projected through an original image, via a projection lens, and focused on the substrate layer of the lenticular sheet. The lenticular images are exposed on a recording material as interdigitated image lines. Japanese (Kokoku) Patent Applications Nos. 42-5473, 48-6488, 49-607, and 53-33847 disclose recording apparatus in which two original images are projected for printing on a lenticular recording material. Recording lenticular images in this fashion (i) requires complex projection lens systems, which are expensive, and (ii) does not work well with thermal dye transfer approaches because it requires more power than what is produced by a halogen lamp or a similar light source.
In contrast, image recording by scanning (linear) exposure requires comparatively simple optics, yet has great flexibility in adapting to various image processing operations, and to alterations in the dimension of the lenticulas. To take advantage of these features, various apparatus and methods have been proposed for recording an image by scanning exposure. For example, Japanese (Kokoku) Patent Application No. 59-3781 teaches a stereoscopic image recording system in which a plurality of original images is taken with a TV camera, processed and stored in frame memories from which the stored image signals are retrieved sequentially as image lines in accordance with the pitch of lenticulas used. After the image lines are recorded on a substrate layer by scanning exposure, the upper layer of the lenticular sheet is bonded to the substrate layer containing the image lines. Another image recording system uses polygon scanners, described in U.S. Pat. No. 5,349,419, for exposure of stereoscopic images directly on photosensitive back surface of a lenticular sheet.
It is desirable to write interdigitated images directly on a back surface of lenticular sheet using thermal dye transfer. This would eliminate the need for careful alignment of specific pre-printed image lines of the substrate layer with the specific lenticulas of the upper layer of the lenticular sheet. The use of thermal dye transfer to write such interdigitated images requires, however, large amounts of energy. Such energy can be provided by high power lasers.
Furthermore, high quality lenticular images require that a large number of images be placed behind a fine pitched lenticular sheet. For example, in order to produce 25 images with a lenticular sheet of 100 lenticulas per inch one needs to produce 2500 lines per inch of continuous tone spots. This means a pixel size of approximately 10 microns or less. To expose such a small pixel, the beam size has to be approximately of the same size as the pixel size. A single mode laser can easily provide such a small beam size. Therefore, a single mode laser, may be used to write interdigitated images. Unfortunately, high power, inexpensive single mode diode lasers are not available.
Inexpensive high power multimode lasers are commercially available. However, such lasers have two major problems. Firstly, they have an emitting aperture with a high aspect ratio and elliptical beam divergence. These characteristics make it hard to obtain a scanning spot with desired size and shape. Secondly, the emitting aperture size of the laser and hence the spot size at the recording material in any direction is inversely proportional to the amount of laser power in this direction. However, it is desired that a laser thermal printer has a high power density, i.e., that it has both the maximum power and the smallest possible spot size. Because a multimode laser produces a spot size that is long (large spot size in one dimension), the laser power is spread across the length of the spot, resulting in low power density.